Why does my for loop return 'undefined' as the last element of a string

Question

I was trying to reverse a string using a for loop. The approach I took was to iterate backwards through the entire string, get each separate character of a string in a descending order and then concatenate the reversed characters together in a new, reversed string. However, I encountered a small issue - the string did reverse, but 'undefined' was concatenated at the start.

What am I doing wrong and why this is hapenning? Many thanks for help in advance.

Please, see the code snippet and console for details.

const string = "A quick brown fox jumped over the lazy dog.";
let newStr = "";

for (let i = string.length; i >= 0; i--) {
  let char = string[i];
  newStr += char;
}

console.log(newStr);

Your i starts at string.length, consider what happens when you try and access string[string.length] from your string — Nick Parsons
– Nick Parsons, Commented Feb 14, 2022 at 11:30
The following makes it simple to reverse. string.split("").reverse().join("") — k22pr
– k22pr, Commented Feb 14, 2022 at 11:37

Mechanic · Accepted Answer · 2022-02-14 11:38:01Z

3

Because array starts from 0 and latest element is accessible by length-1 so the array[length] is equal to writing array[999999]

const string = "A quick brown fox jumped over the lazy dog.";
let newStr = "";

for (let i = string.length-1; i >= 0; i--) {
  let char = string[i];
  newStr += char;
}

console.log(newStr);
//alternative way
console.log(string.split('').reverse().join(''));

edited Feb 14, 2022 at 11:38

answered Feb 14, 2022 at 11:30

Mechanic

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2 Comments

superpav Over a year ago

Makes sense, thank you. I actually knew how to get the last element in an array/string, just couldn't put 2 and 2 together.

Mechanic Over a year ago

@superpav cool, I'd also added an alternative way FYI too

Ran Turner · Accepted Answer · 2022-02-14 11:42:02Z

0

You should start from string.length - 1 since string[string.length] is trying to refer an element out of bounds of the array, hence, evaluates to undefined

const string = "A quick brown fox jumped over the lazy dog.";
let newStr = "";

for (let i = string.length - 1; i >= 0; i--) {
  let char = string[i];
  newStr += char;
}

console.log(newStr);

answered Feb 14, 2022 at 11:42

Ran Turner

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